Fossil footprints show dinosaurs and early mammals living side-by-side

A sandstone slab found on the grounds of NASA's Goddard Space Flight Center contains fossilized footprints from both dinosaurs and mammals from about 100 million years ago(Credit: NASA Goddard Space Flight Center)

A new discovery has been made at NASA's Goddard Space Flight Center in Greenbelt, Maryland, that has nothing to do with space. A sandstone slab covered in fossilized footprints has been excavated from the grounds, giving us a glimpse into a day-in-the-life of the area about 100 million years ago. The slab contains 70 tracks from at least eight different species, including rarely-seen interactions between both dinosaurs and mammals.

Dusty old bones can only teach us so much about prehistoric creatures. Fossilized footprints, on the other hand, can show us how the animals moved and interacted. In recent years, paleontologists have discovered the tracks of an unknown "mega-carnivore" in Africa, a patch in Western Australia that includes prints from 21 different dinosaur species, and a double record-breaking set in France, made up of the longest set of tracks made by the largest sauropod.

While the dinosaur tracks on the Goddard slab are impressive, it's the other prints that really make it special. Fossilized mammal footprints from that time are relatively rare, and it's even rarer to see them weaving between dino tracks.

Measuring about 8 ft (2.4 m) long and 3 ft (0.9 m) wide, the Goddard slab contains more than 70 tracks, at least 26 of which are mammalian. Among those creatures leaving their mark along this walk of fame are huge long-necked sauropods, tank-like nodosaurs, crow-sized carnivorous therapods, flying pterosaurs, and mammals about the size of squirrels.

"The concentration of mammal tracks on this site is orders of magnitude higher than any other site in the world," says Martin Lockley, co-author on a study describing the find. "I don't think I've ever seen a slab this size, which is a couple of square meters, where you have over 70 footprints of so many different types. This is the mother lode of Cretaceous mammal tracks."

The scientists believe the tracks would have all been made within the space of a few days, and it was apparently a high-traffic area, likely along the edge of a wetland. The interactions between the animals is preserved in astonishing detail, with researchers picking out a baby nodosaur walking alongside a parent, parallel theropod tracks that indicate they were hunting as a group, and pairs of mammal tracks that show evidence of the creatures pausing to sit back on their haunches.

"It's a time machine," says Ray Stanford, the initial discoverer of the tracks and co-author of the study. "We can look across a few days of activity of these animals and we can picture it. We see the interaction of how they pass in relation to each other. This enables us to look deeply into ancient times on Earth. It's just tremendously exciting."

The Goddard slab is also home to the largest known individual mammal footprint from that era. Measuring about 4 sq in (26 sq cm), the print was likely left by an animal about the size of a racoon – not huge by today's standards, but bigger than the usual squirrel- and rat-sized mammals that commonly scurried under the feet of dinosaurs.

Stanford discovered the prints in 2012, after dropping off his wife at work at the Goddard Space Flight Center. The slab was later excavated and studied, and a cast was made.

"This could be the key to understanding some of the smaller finds from the area, so it brings everything together," says Lockley. "This is the Cretaceous equivalent of the Rosetta stone."

The study was published in the journal Scientific Reports. The team discuss the slab in the video below.

A sandstone slab found on the grounds of NASA's Goddard Space Flight Center contains fossilized footprints from both dinosaurs and mammals from about 100 million years ago(Credit: NASA Goddard Space Flight Center)

The Goddard slab contains more than 70 individual tracks left by eight different species, ranging from huge sauropods to tiny mammals(Credit: NASA Goddard Space Flight Center)

Measuring about 4 sq in (26 sq cm), this print (lower right) is the largest known mammal footprint from that long ago, left by an animal about the size of a racoon(Credit: NASA Goddard Space Flight Center/<a href="https://creativecommons.org/licenses/by/2.0/" rel="nofollow">Creative Commons (CC BY 2.0)</a>)

Pairs of mammal tracks show evidence that the creatures paused to sit back on their haunches(Credit: NASA Goddard Space Flight Center/<a href="https://creativecommons.org/licenses/by/2.0/" rel="nofollow">Creative Commons (CC BY 2.0)</a>)